Audio signals can possess one or more components: speech, music, noise, natural sounds, synthetic sounds and/or any audio signal with the same characteristics, components which are digitally processed in view of the various digital multimedia applications, such as for example digital television, DVD's, records, music CD's, Internet services, interactive multimedia services. There are a great many mathematical methods for processing the audio signal. It is customary to use frequency and temporal transformations, prediction or statistical algorithms, mechanisms for producing sound and speech, acoustic analysis and mechanisms using the ear's properties of perception.
For example, the speech coders are based on its statistical characteristics, such as variance and auto correlation, which give rise to predictive and adaptive algorithms, likewise on its spectral properties (pitch (relative to fundamental), formants (related to the spectral enclosure), voicing, non-voicing). Numerous algorithms likewise exist in the frequency, temporal, parametric, and analysis and synthesis coding domains.
For various digital applications, more and more reliable modeling, quantification, compression and transmission means have been perfected and have given rise to many audio coders with better and better performance in terms of quality, compression, cost and reliability. For example, the MPEG-AAC (Motion Picture Expert Group-Advance Audio Coding) is currently considered to be the standard in compression of Hi-Fi band audio signals compression that is most efficient and most universal. Nevertheless, as more and more multimedia applications are offered on the market, they are also very often pirated.
From prior practice we already know about a security system for portable music players, through WO 0058963 (Liquid Audio). Data such as a musical piece are saved both as a portable secured piece (SPT: secure portable track), which could be connected to one or more readers (“players”) and could be connected to a specific means of data backup, thus restricting playing the SPT to specific players and ensuring that playing takes place only from the original means of backup. The SPT is connected to a player by encrypting of SPT data by using a backup key which is unique to the player, difficult to change and is kept by the player under strict security conditions. The SPT is connected to a specific means of backup by including data that uniquely identifies the backup means in a form that is resistant to falsification, i.e. signed in an encrypted manner.
U.S. Pat. No. 4,600,941 (Sony) discloses a scrambling system for audio signals in which an audio signal is divided into blocks, where each block consists of multiple fields, where the multiple fields are rearranged on a time base in a n order that is predetermined for each block such that they are encoded and the encoded signal is re-arranged on a time basis in an original order such that it can be decoded, in which a first signal processing circuit is provided for inserting a redundant portion into a portion between contiguous fields and compress the fields in base time in response to the redundant portions during encoding, where a circuit generates a signal for inserting a monitoring signal other than a piece of audio information into the redundant portions, a signal monitoring detection circuit for detecting the monitoring signal during decoding and a second circuit for processing the signal for removing the redundant portions in synchrony with the detected monitoring signal and decompressing the fields in base time in response to the redundant portions.
U.S. Pat. No. 5,058,159 (Macrovision Corporation) discloses a means and a system for scrambling and unscrambling audio information signals. The audio signals are scrambled by inversing the original frequency specter such that the portions of the frequency that originate below in the audio frequency band are shifted upwards whereas the portions originating above the band are shifted downwards. A pilot sound of known frequency is recorded with the audio signals at the shifted frequencies. During reproduction, each phase and frequency variation is sought out by the pilot sound, which is used to generate the demodulation signal to reconstitute the original frequency content of the audio signals.
WO 00 55089 A discloses a means and a system for scrambling digital samples which may or may not be compressed, representing audio and video data, such that the contents of these samples are degraded, but recognizable, or otherwise provided with a required given quality. A given number of LSBs (“Least Significant Bits”, lightest weight bits) data are scrambled for each sample field by field, in an adaptive manner as a function of the dynamic of possible values, where the highest weight bits are unchanged. This solution represents an encrypting solution that is well known to the craftsman, using (an) encrypting key(s). The encrypting keys are transmitted all at once or entirely in the flux with the encrypted data, which makes the flux vulnerable to attempts at pirating, given that all the elements comprising the audiovisual flux remain inside the flux. However, it does not provide the desired high security.
DE 199 07 964 C discloses a device used to generate an encrypted data flux which represents an audio and/or video signal. This prior art develops means and techniques for protecting the audio (and/or video) flux by modifying, using one or more keys, certain information in the original flux, for example encrypting is carried out by modifying the LSB's (“Least Significant Bits”, lightest weight bits) of the spectral coefficients.
Given that protection is carried out using encrypting keys, all the initial information remains present inside the protected flux. However, it too does not provide the high security criteria.
The state of the art gives proof of many audio flux protection systems, which are essentially based on encryption of data, by adding encrypting keys that are independent of the audio flux content, and which therefore modify the format of the structured flux. A specific and different embodiment is that of the Coding Technologies company, which consists of using scrambling to protect a selected part of the bitstream (“bitstream” is the name for the binary flux at the output of the audio encoder) and not the entire bitstream. The protected parts represent the spectral values of the audio signal, which means that during decoding without unencrypting, the audio flux is distorted and unpleasant to listen to.
It would therefore be advantageous to provide a means that makes reconstitution of a modified audio flux impossible to ensure audio protection of any broadcast system whatsoever (audio or audiovisual).